1. Field of the Invention
The present invention relates to a method for producing a capping wafer for a sensor, in particular a sensor in a motor vehicle. In addition, the present invention relates to a method for producing a sensor stack, in particular an inertial sensor stack, having at least one sensor. In addition, the present invention relates to a capping wafer, to a sensor having a cap according to the present invention, and to a configuration of a substrate having a sensor according to the present invention.
2. Description of Related Art
A sensor, in particular a microelectromechanical sensor (MEMS sensor), in particular a capacitive inertial sensor for measuring accelerations and/or rates of rotation, is as a rule packaged in a plastic housing together with an IC chip that evaluates and pre-processes a raw sensor signal. Before installing the sensor in the plastic housing, a sensitive sensor structure is provided with a cap. This is necessary in order to protect movable sensor structures from subsequent processes, including for example gelation, and in order to permit the setting of a desired internal pressure and thus a desired sensor oscillation quality.
For this purpose, before the separation of a large number of (capacitive inertial) sensors, a so-called capping wafer is bonded onto an electrical sensor wafer using sealing glass bonds. This results in a so-called sensor stack that comprises the sensor wafer and the capping wafer fastened thereon, forming a large number of sensors bonded fixedly to one another. Here, the sealing glass is applied in the shape of a frame around caverns of the capping wafer using a screen printing method. The sealing glass (glass solder) ensures a hermetic seal between the capping wafer and the sensor wafer in the region of the caverns.
The job of the caverns is to keep small a relative change of an enclosed volume between the capping wafer and the sensor wafer when the sealing glass is pinched, and to act as a pressure or vacuum reservoir for the sensor, which will later be separated. The accesses to the electrical terminals (bonding pads) of each sensor are situated outside the frame, and in the capping wafer are already recessed in a suitable manner and thus made accessible. See here for example published German patent document DE 197 00 734 A1.
After the separation of the sensor stack, the individual sensors can be glued into the plastic housing. The electrical connections from the sensor to the IC chip and the electrical connections from the electrical terminals of the housing to the bonding pads of the IC chip are realized using bonding wires. Subsequently, the bonding layers are gelated and the plastic housing is sealed. Only then is the resulting sensor element ready to be soldered onto a circuit board. In sensors having a hybrid construction (two-chip solution having a sensor chip and an IC chip), such a procedure is required because the electrical connections, which are high-ohmic and are susceptible to disturbances, between the (capacitive) sensor and an evaluation electronics system must be realized in a defined manner, mechanically fixed, and protected.
U.S. Pat. No. 7 275 424 B2 discloses a manufacturing method for a sensor stack, as well as a sensor stack, or a manufacturing method for a capped sensor, as well as a sensor. Here, in a capping wafer caverns are provided, and outside a cavern electrically insulated vias are provided. Subsequently, the capping wafer is further processed, by a sealing glass bonding with a sensor wafer, to form a sensor stack, the sealing glass being applied onto the capping wafer using a screen printing method, and the sealing glass bond between the capping wafer and the sensor wafer being formed subsequently. After this, the vias of the sensor stack are filled with a doped polysilicon material or by chemical nickel-plating in such a wave that an electrical contacting results to aluminum contacts of the sensor wafer that are situated inside the sensor stack. Subsequently, an intermediate space situated between the capping wafer and the sensor wafer is filled with a polymer, an external side on the capping wafer of the sensor stack is provided with an electrical insulating layer, and subsequently an electrical outer contacting of the sensor stack to the outer insulating layer is provided. For this purpose, the insulating layer in the area of the internal electrical contacting is etched free to the inner aluminum contacts and is filled with a metal that extends up to the external side of the insulating layer, forming a plate. A solder bump is placed on each such plate in order to enable the sensors to be soldered onto a circuit board.
Here it is problematic that an electrical through-connection of the sensor stack is not set up until after bonding of the sensor wafer to the capping wafer. If a process of filling the vias is not successful, the entire sensor stack, or at least a part thereof, must be scrapped. Furthermore, in this method it is not possible to produce an individual capping wafer having a through-connection, so that such a method also cannot be outsourced to a supplier. In addition, it is not possible to fill the vias using an electroplating method, so that the selection of a material for the through-connection, and for a method of filling it, is limited.